JP5821336B2 - Stainless steel for polymer electrolyte fuel cell separator, method for producing the same, and polymer electrolyte fuel cell separator - Google Patents

Description

  The present invention relates to a stainless steel for a polymer electrolyte fuel cell separator that is industrially stable and has excellent electrical conductivity, a method for producing the same, and a polymer electrolyte fuel cell separator.

In recent years, development of fuel cells that are excellent in power generation efficiency and do not emit CO ₂ has been promoted from the viewpoint of global environmental conservation. There are several types of fuel cells, such as phosphoric acid type fuel cells and solid electrolyte type fuel cells, depending on the type of electrolyte used. Among them, the polymer electrolyte fuel cell can be operated at a low temperature of 100 ° C. or less, can be started in a short time, and is suitable for downsizing. Therefore, a stationary power generator for home use and a power source for mounting a fuel cell vehicle It is used for such as.

  In a polymer electrolyte fuel cell, necessary electric power is obtained by stacking a number of cells each having a polymer electrolyte membrane sandwiched between separators. A thin material having a thickness of several tens to several hundreds μm is used for the separator so that the thickness when laminated is reduced. Because this separator requires good electrical conductivity and corrosion resistance at high potential, graphite is used, but graphite is vulnerable to impact, and it takes time to process the flow path for flowing hydrogen etc. There is. Therefore, application to a stainless steel separator that is resistant to impact and easy to process has been studied.

  However, since a passive film is formed on the surface of stainless steel, the surface contact resistance is high, and as such, it is difficult to use it as a fuel cell separator. Therefore, it has been studied to reduce the surface contact resistance by modifying the surface of the stainless steel passive film by dipping it in an acid solution.

  In Patent Document 1, the composition of steel is C ≦ 0.03%, N ≦ 0.03%, 20% ≦ Cr ≦ 45%, and 0.1% ≦ Mo ≦ 5.0% in mass%. A stainless steel for a polymer electrolyte fuel cell separator is disclosed, wherein the Cr / Fe atomic ratio Cr / Fe contained in the dynamic film is 1 or more. In the present invention, the passivation film having the above characteristics is formed by surface modification using an acid solution containing hydrochloric acid and nitric acid or an acid solution containing hydrofluoric acid and nitric acid. However, when surface modification treatment is continuously performed with the same acid solution, the immersion time required for the modification of the passive film increases because metal ions such as Fe are mixed into the acid solution due to the dissolution of stainless steel. However, there is a problem that it is difficult to carry out industrial continuous processing.

  The stainless steel used for the fuel cell separator is in the form of a foil, but a general stainless steel foil is manufactured by adjusting the surface properties by performing bright annealing with a dew point of −60 to −50 ° C. Also in Patent Document 1, a BA-finished cold-rolled stainless steel sheet that has been annealed (850 to 1050 ° C.) in an ammonia decomposition gas having a dew point of −60 ° C. is used.

Patent Document 2 discloses that the residence time in the temperature range from room temperature to 800 ° C. in a reducing atmosphere gas adjusted to a dew point of −45 ° C. or lower is 5 to 60 seconds, and then the dew point −40 in the temperature range of 800 to 1150 ° C. A bright annealing is disclosed in which the residence time in a reducing atmosphere gas adjusted to below C is 30 to 180 seconds.
Patent Document 3 discloses bright annealing in which the hydrogen gas in the atmosphere is 70% by volume or more, the balance is substantially made of nitrogen gas, and the dew point of the atmosphere gas is -40 ° C. or less.

  However, in any of the bright annealing methods disclosed in Patent Documents 2 and 3, when metal ions such as Fe are mixed in the acid solution described above, it is difficult to industrially perform continuous treatment, which is necessary. The problem of increased immersion time cannot be solved.

  As described above, when the surface modification treatment is performed after performing the bright annealing disclosed in the past, the electrical conductivity necessary for increasing mainly the Fe ion concentration in the acid solution is obtained as the treatment amount increases. There is a problem that it becomes difficult.

JP 2004-149920 A Japanese Patent Laid-Open No. 2005-213589 JP 2008-1945 A

  The present invention relates to a stainless steel that can provide industrially stable and excellent electrical conductivity for a polymer electrolyte fuel cell separator having excellent electrical conductivity, a method for producing the same, and a polymer electrolyte fuel cell separator. The purpose is to provide.

In order to solve the above-mentioned problems and to obtain stainless steel having excellent electrical conductivity, the present inventors have conducted heat treatment conditions and surface modification treatment of cold-rolled sheets (cold-rolled steel sheets) and cold-rolled annealed sheets. By examining and combining specific heat treatment and surface modification treatment, it was found that stable production of stainless steel having excellent electrical conductivity was possible.
That is, this invention makes the following structure a summary.
[1] By mass%, C: 0.001 to 0.10%, Si: 0.001 to 1.0%, Mn: 0.001 to 1.2%, Al: 0.001 to 0.5% , Cr: 15.0 to 35.0%, N: 0.001 to 0.10%, the balance is made of Fe and inevitable impurities, and the thickness of the oxide film on the surface is 20 to 600 nm Features stainless steel.
[2] Further, by mass%, Ti: 1.0% or less, Nb: 1.0% or less, Zr: 1.0% or less, Cu: 1.0% or less, V: 1.0% or less, Ni The stainless steel as set forth in [1], containing at least one of 12.0% or less and Mo: 5.0% or less.
[3] The stainless steel according to [1] or [2], wherein an atomic ratio of Si, Al, Mn, and Fe contained in the oxide film satisfies (Si + Al + Mn) /Fe≦1.0.
[4] A stainless steel having the chemical composition according to [1] or [2], and having a surface contact resistance of 20 mΩ · cm ² or less.
[5] A polymer electrolyte fuel cell separator comprising the stainless steel according to [4].
[6] When producing the stainless steel according to any one of [1] to [3] , after cold rolling or after annealing the cold rolled material, the hydrogen concentration is 30% by volume or more and the remainder is not present. A method for producing stainless steel, comprising performing a heat treatment at a temperature of 800 to 1200 ° C in an atmosphere comprising an active gas and inevitable impurities and having a dew point of -40 to 0 ° C.
[7] When producing the stainless steel according to the above [4], after cold rolling or after annealing the cold rolled material, the hydrogen concentration is 30% by volume or more, and the balance consists of inert gas and inevitable impurities, under an atmosphere dew point is -40～0 ° C., the temperature is subjected to heat treatment at 800 to 1200 ° C., after the heat treatment, a method of manufacturing features and to Luz stainless steel to make a surface modification treatment using an acid solution .
[8] The method for producing stainless steel according to [7], wherein the acid solution is an inorganic acid.
[9] The method for producing stainless steel according to [8], wherein the inorganic acid is hydrofluoric acid or hydrofluoric acid.

  According to the present invention, even when metal ions such as Fe are mixed in the acid solution by dissolving stainless steel, the stainless steel that provides industrially stable and excellent electrical conductivity, its production method, and solid high A molecular fuel cell separator is obtained.

  In addition, according to the method for producing stainless steel of the present invention, it is possible to produce a stainless steel separator by stable continuous treatment, industrial mass production can be performed, and the treatment equipment is largely maintained. The costly electrolytic treatment is not necessary, and the treatment of hexavalent chromium produced by the electrolytic treatment is unnecessary, so that the burden on the environment is reduced.

Effect of immersion time in surface modification treatment on surface contact resistance of each oxide film Effect of oxide film thickness on immersion time in surface modification necessary to reduce surface contact resistance Change in surface contact resistance depending on measurement position

  The present invention is described in detail below.

Components of stainless steel The reasons for limitation of each component element will be described below. Here, “%” notation indicating the content of the component means “% by mass” unless otherwise specified.

C: 0.001 to 0.10%
C is an element inevitably contained in stainless steel, and has the effect of increasing the strength of the steel by solid solution strengthening. The effect cannot be obtained at less than 0.001%. On the other hand, excessive inclusion promotes precipitation of Cr carbide, locally decreases the Cr content of the surrounding iron around the Cr carbide, and lowers the corrosion resistance of stainless steel. The effect becomes remarkable when it exceeds 0.10%. Therefore, C is set to 0.001 to 0.10%. More preferably, it is 0.002 to 0.04%.

Si: 0.001 to 1.0%
Si is an element useful for deoxidation, and the effect is obtained at 0.001% or more. However, excessive inclusion promotes concentration into an oxide film and inhibits surface contact resistance reduction by surface modification treatment. This tendency becomes remarkable at 1.0% or more. Therefore, Si was 0.001 to 1.0%. More preferably, it is 0.005 to 0.2%.

Mn: 0.001 to 1.2%
Mn is an element inevitably mixed in the steel and has an effect of increasing the strength of the steel. The effect is obtained at 0.001% or more. However, since MnS is precipitated and becomes a starting point of corrosion, excessive content lowers the corrosion resistance. Further, concentration in the oxide film inhibits surface contact resistance reduction by surface modification treatment. The tendency becomes remarkable at 1.2% or more. Therefore, Mn is set to 0.001 to 1.2%. More preferably, it is 0.005 to 0.2%.

Al: 0.001 to 0.5%
Al is an element useful for deoxidation, and the effect is obtained at 0.001% or more. However, excessive inclusion inhibits surface contact resistance reduction by surface modification treatment when an oxide film is formed. This tendency becomes remarkable at 0.5% or more. Therefore, Al was made 0.001 to 0.5%. More preferably, it is 0.005 to 0.2%.

Cr: 15.0-35.0%
Cr is an element important for the corrosion resistance of stainless steel, and is an element that improves the corrosion resistance as the content increases. In order to ensure sufficient corrosion resistance in the usage environment of the fuel cell separator, the content is desirably 15.0% or more. On the other hand, if the Cr content exceeds 35.0%, the passive film becomes strong and the surface contact resistance tends to increase, so that it is inappropriate for use in a fuel cell separator. Therefore, the content of Cr is set to 15.0 to 35.0%. More preferably, it is 18.0 to 31.0%.

N: 0.001 to 0.10%
N, like C, is an element inevitably contained in stainless steel, and has the effect of increasing the strength of the steel by solid solution strengthening. Furthermore, it has the effect of improving corrosion resistance by dissolving in steel. Those effects cannot be obtained at less than 0.001%. On the other hand, when Cr nitride is deposited, the corrosion resistance of the stainless steel is lowered. The effect becomes remarkable when it exceeds 0.10%. Therefore, C is set to 0.001 to 0.10%. More preferably, it is 0.002 to 0.04%.

  The balance is Fe and inevitable impurities, but for the following reasons, Ti: 1.0% or less, Nb: 1.0% or less, Zr: 1.0% or less, Cu: 1.0% or less, V : 1.0% or less, Ni: 12.0% or less, Mo: It is preferable to contain 1 or more types among 5.0% or less.

Ti: 1.0% or less Ti is an element that preferentially bonds with C and N and suppresses the deterioration of corrosion resistance due to the precipitation of Cr carbonitride. On the other hand, when it exceeds 1.0%, the workability is lowered, and Ti carbonitride is coarsened to cause surface defects. Therefore, Ti is set to 1.0% or less.

Nb: 1.0% or less Nb is an element that preferentially binds to C and N and suppresses a decrease in corrosion resistance due to the precipitation of Cr carbonitride. On the other hand, if it exceeds 1.0%, the hot strength increases and the hot rolling load increases, which makes manufacturing difficult. Therefore, Nb was made 1.0% or less.

Zr: 1.0% or less Zr is an element that binds preferentially to C and N and suppresses a decrease in corrosion resistance due to the precipitation of Cr carbonitride. On the other hand, if it exceeds 1.0%, the workability decreases. Therefore, Zr is set to 1.0% or less.

Cu: 1.0% or less Cu is an element that improves the corrosion resistance of stainless steel. However, excessive content increases the elution of metal ions and reduces the corrosion resistance of stainless steel. The tendency becomes remarkable when it exceeds 1.0%. Therefore, Cu was made 1.0% or less.

V: 1.0% or less V is an element that improves the corrosion resistance of stainless steel. However, if the content exceeds 1.0%, the workability is lowered and the molding process of the separator becomes difficult. Therefore, V is set to 1.0% or less.

Ni: 12.0% or less Ni is an element that suppresses active dissolution and improves the corrosion resistance of stainless steel. However, if it exceeds 12.0%, the overpassive dissolution is promoted, and the corrosion resistance in the overpassive range is lowered. Therefore, Ni was made 12.0% or less.

Mo: 5.0% or less Mo is an element that promotes repassivation of the passive film damaged by scratches and the like, and improves the corrosion resistance of stainless steel. However, addition exceeding 5.0% increases the strength and increases the rolling load, making it difficult to produce. Therefore, Mo is set to 5.0% or less.

  In addition, W is 1.0% or less for the purpose of improving corrosion resistance, and Ca, Mg, REM (Rare Earth Metals) and B are each 0.1% for the purpose of improving hot workability. It can also be contained below.

  Of the inevitable impurities, Sn is preferably less than 0.001% and O is preferably 0.02% or less.

The atomic ratio of Si, Al, Mn, and Fe contained in the oxide film Si, Al, and Mn are elements that are concentrated in the oxide film by heat treatment such as bright annealing. The oxides of these elements are oxides of Fe More stable and less susceptible to acid. Therefore, if oxides of these elements are present in a large amount in the oxide film, it becomes difficult to dissolve the oxide film. On the other hand, Fe oxide is relatively easy to dissolve in an acid solution. In addition, since an oxide of Fe has a high generation rate and is likely to have a rough structure, ions and an acid solution are easily transmitted, and the effect of protecting stainless steel from the acid solution is small.

  As a result of analysis by photoelectron spectroscopy (XPS) on several oxide films formed on the surface of stainless steel by various methods, the atomic ratio of Si, Al, Mn, Fe contained in the oxide film is (Si + Al + Mn). When it was /Fe≦1.0, there was a tendency to be easily dissolved in the acid solution. Therefore, the atomic ratio of Si, Al, Mn, and Fe contained in the oxide film was set to (Si + Al + Mn) /Fe≦1.0. Here, the atomic ratio of Si, Al, Mn, and Fe is obtained by performing analysis by photoelectron spectroscopy (XPS).

Oxide Film Thickness As described above, general stainless steel has an oxide film or a passive film on the surface of several nanometers. Therefore, it is preferable to reduce the surface contact resistance by surface modification treatment with an acid solution.

  The inventors have determined that the thickness of the oxide film for obtaining stainless steel that is industrially stable and provides excellent electrical conductivity, and the immersion time required for reducing the surface contact resistance with an acid solution mixed with metal ions. The relationship was investigated.

  Stainless steel having the composition shown in steel type symbol A in Table 1 was vacuum-melted and heated to 1250 ° C., and then hot-rolled, hot-rolled sheet annealed (1000 ° C.), and pickled the hot-rolled sheet. Furthermore, cold rolling, cold-rolled sheet annealing (950 ° C.), pickling of the cold-rolled sheet is performed, and 800 to 1200 in an ammonia decomposition gas (75% by volume of hydrogen—25% by volume of nitrogen) at a dew point of −60 to 10 ° C. A heat treatment was performed at a temperature of 30 to 180 s at a soaking time to obtain a stainless steel foil having a thickness of 0.3 mm.

The thickness of the oxide film of the produced stainless steel foil was measured by Auger electron spectroscopy (AES). The thickness of the oxide film was obtained by multiplying the sputtering time at which the O concentration was half of the maximum value by the sputtering rate measured with SiO ₂ . Note that the sputtering rate of SiO ₂ is 3 nm / min. The produced stainless steel foil had an oxide film thickness of 5 to 720 nm.

These stainless steel foils were surface-modified by immersing them in an acid solution containing 1 g / L Fe dissolved in 5% HNO ₃ -20% HF hydrofluoric acid for 30 to 1200 s, and then 10 mm square carbon at a pressing pressure of 1 MPa. The surface contact resistance was measured by pressing the paper. For use as a fuel cell separator, a surface contact resistance of 20 mΩ · cm ² or less is acceptable.

  FIG. 1 shows the influence of the immersion time on the surface contact resistance of samples having an oxide film thickness of 5 nm, 110 nm, and 380 nm before the surface modification treatment. As the immersion time increased, the surface contact resistance decreased, but the slope and the immersion time required to reach the pass line differed depending on the thickness of the oxide film.

FIG. 2 shows the influence of the thickness of the oxide film on the immersion time necessary to reduce the surface contact resistance. The minimum immersion time required for the surface contact resistance to be 20 mΩ · cm ² or less is shown for stainless steel foils with various oxide film thicknesses. The immersion time required for the surface modification treatment had a convex correlation with the thickness of the oxide film being minimized at around 120 nm. The immersion time that allows industrially continuous treatment was about 300 s, and the required immersion time was less than 300 s when the oxide film thickness was in the range of 20 to 600 nm. Therefore, it is considered that industrial continuous processing is possible if the stainless steel has an oxide film thickness in this range.

  When the structure of the oxide film was analyzed by thin film X-ray diffraction, when the oxide film thickness was 120 nm or less, there was a tendency that the full width at half maximum of the peak indicating each crystal structure increased as the thickness decreased. It is presumed that the oxide film has changed from a crystalline one to an amorphous one as the thickness decreases. At 120 nm or more, no significant difference was observed in the half width.

  The required immersion time increases when the oxide film is thin. In addition to forming a dense oxide film with few defects and cracks, the oxide forming the oxide film becomes amorphous like the passive film. This is considered to be because it is difficult to dissolve or alter with an acid solution because it inhibits permeation of hydrogen ions and metal ions. When the thickness of the oxide film increases from this state, the formed oxide film has a rough structure, and voids and cracks increase. Through such defects, hydrogen ions, metal ions, or the acid solution itself is generated. Easy to penetrate.

  Therefore, dissolution of stainless steel is promoted by increasing the thickness of the oxide film. Furthermore, as the thickness of the oxide film increases, the quality of the oxide film does not change significantly, but it takes time for ions and acid solutions to penetrate the oxide film simply because the thickness has increased. As a result, the time required for dissolution and alteration of the oxide film increases. For the above reasons, it is estimated that a convex correlation appears in the thickness of the oxide film and the immersion time.

  The upper limit of the immersion time that allows industrially continuous treatment is about 300 seconds. The immersion time required for reducing the surface contact resistance was 300 s or less when the thickness of the oxide film was in the range of 20 to 600 nm. Therefore, the thickness of the oxide film was set to 20 to 600 nm. More preferably, the immersion time is 50 to 500 nm at which the immersion time is 200 s or less. More preferably, it is more than 100 nm to 400 nm.

Surface contact resistance The surface contact resistance is measured by pressing 10 mm square carbon paper against a stainless steel surface at a pressing pressure of 1 MPa. In order to use as a fuel cell separator, the surface contact resistance is preferably ²⁰ mΩ · cm ² or less in order to ensure the power generation efficiency of the fuel cell. More preferably, it is 10 mΩ · cm ² or less.

Manufacturing method of stainless steel After heating stainless steel having the above chemical composition to 1100 to 1300 ° C, the finish rolling temperature is 700 to 1000 ° C, the coiling temperature is 400 to 700 ° C, and the plate thickness is hot to 2.0 to 5.0 mm. Roll. The hot-rolled steel strip thus produced is annealed by hot-rolled sheet at a temperature of 800 to 1200 ° C. and pickled hot-rolled sheet, and then cold-rolled and cold-rolled sheet (cold-rolled sheet) are subjected to multiple annealing. Repeatedly, the foil thickness is 0.03 to 0.3 mm. The temperature of the cold-rolled sheet annealing may be 800 to 1100 ° C., and the cold-rolled sheet may be pickled after the cold-rolled sheet annealing. When performing cold-rolled sheet annealing, it is preferable to perform the dew point of -40 ° C. or less with an atmosphere gas composition containing hydrogen. Thereafter, heat treatment and surface modification treatment described below are performed.

Composition of heat treatment atmosphere By containing hydrogen in the heat treatment atmosphere, it is possible to obtain a reducing action and to suppress the formation of a thick oxide film on the stainless steel surface by the heat treatment. The inventors have found that by appropriately controlling the thickness of the oxide film formed by the heat treatment, the oxide film can easily penetrate into the acid solution. When the hydrogen concentration is less than 30% by volume, it is difficult to obtain a stable reducing action due to slight fluctuations in the dew point and the heat treatment temperature, and it becomes difficult to control the properties of the oxide film. Therefore, the hydrogen concentration in the heat treatment atmosphere is set to 30% by volume or more. More preferably, the hydrogen concentration is 50% by volume or more. The balance is nitrogen and inevitable impurities, but other inert gases that do not contribute to the oxidation of stainless steel can be used instead of nitrogen. The inert gas here refers to nitrogen and rare gases such as helium, neon, and argon.

Dew point of heat treatment atmosphere The lower the dew point, the less moisture contained in the furnace gas, indicating a more reducing atmosphere. In bright annealing, which is a general heat treatment, the dew point is −60 to −50 ° C., and a very thin oxide film having a thickness of several nm is formed. Since an oxide film is more easily formed in an oxidizing atmosphere, the thickness of the oxide film becomes thicker. When the dew point is less than −40 ° C., the growth of the oxide film is slow, and it becomes difficult to control the thickness of the oxide film. If the dew point is 0 ° C. or higher, the thickness of the oxide film becomes thick, and partial abnormal oxidation occurs to impair the surface uniformity. Therefore, the dew point was set to -40 to 0 ° C. More preferably, it is -30-30 degreeC, More preferably, it is -20-0 degreeC.

  In addition, Al, Si, Mn, and Fe are easily oxidized in this order, and in an atmosphere where the reducibility is strong to some extent, the oxide of Fe is reduced, so the concentration of Fe atoms in the oxide film decreases, and relative In particular, the atomic number concentration of Al, Si, and Mn increases. Since the atomic number concentration and the atomic number of each element are proportional, in order to make the atomic ratio (Al + Si + Mn) / Fe 1.0 or less, it is preferable that the atmosphere has an appropriate oxidizing property. Therefore, the atmosphere was set such that the hydrogen concentration was 30% by volume or more and the dew point was −40 to 0 ° C.

Heat treatment temperature The higher the heat treatment temperature, the more the oxidation reaction is promoted between the stainless steel and the furnace atmosphere gas. If it is less than 800 ° C., sufficient growth of the oxide film cannot be expected, and if it exceeds 1200 ° C., abnormal oxidation occurs. Therefore, the heat treatment temperature is preferably 800 to 1200 ° C. A more preferable heat treatment temperature is 960 to 1100 ° C.

  In addition, the time for maintaining the temperature of the heat treatment is preferably 10 to 200 s from the viewpoint of manufacturability in a continuous annealing furnace.

In order to stably reduce the surface contact resistance regardless of the metal ion concentration in the acid solution, stainless steel that has controlled the oxide film on the surface of the steel sheet by an appropriate heat treatment becomes an appropriate acid solution. It is necessary to immerse. The oxide film formed under the heat treatment conditions of the present invention is mainly composed of oxides of Fe and Cr, and has a coarser structure than the passive film, so that the acid solution penetrates and reaches the base iron. It's easy to do. Since the oxide forming the oxide film is less soluble in the acid solution than the base iron, the oxide film is removed in such a manner that the oxide film peels off due to the dissolution of the base iron.

Therefore, the acid solution used for the surface modification treatment preferably contains an inorganic acid that is excellent in reactivity with the metal. Inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, and mixtures thereof. Hydrofluoric acid or hydrofluoric acid is particularly preferable because surface contact resistance can be more stably reduced. When hydrofluoric acid (HF) or hydrofluoric acid (HF + HNO ₃ ) is used as the inorganic acid, the concentration of hydrofluoric acid is preferably 2 to 25% and the concentration of nitric acid is preferably 0 to 20%. Here, “%” indicating the concentration of the acid solution means “mass%” unless otherwise specified.

  The temperature of the acid solution is preferably 40 to 85 ° C. from the viewpoints of reactivity and safety. More preferably, it is 40-70 degreeC. In order to carry out the continuous treatment efficiently, the immersion time in the acid solution is preferably 10 to 300 seconds. More preferably, the immersion time in the acid solution is 20 to 200 s.

  Nine kinds of stainless steels having the compositions shown in Table 1 with steel type symbols A to I were vacuum-melted and heated to 1250 ° C., followed by hot rolling, hot-rolled sheet annealing (1000 ° C.), and pickling hot-rolled sheets. went. Furthermore, cold rolling, cold-rolled sheet annealing (950 ° C.), pickling of the cold-rolled sheet is performed, and at 1020 ° C. in ammonia decomposition gas (75% by volume of hydrogen—25% by volume of nitrogen) at a dew point of −60 to 10 ° C. Heat treatment was performed for a soaking time of 100 s to obtain a stainless steel foil having a plate thickness of 0.3 mm. Table 2 shows the heat treatment conditions.

The surface of test samples 1 to 16 in Table 2 was subjected to surface modification treatment by immersing for 300 s in an acid solution in which 1 g / L Fe was dissolved in 5% HNO ₃ -20% HF nitric acid at 60 ° C. Contact resistance was measured. The surface contact resistance was measured by pressing a square carbon paper having a piece of 10 mm with a pressing pressure of 1 MPa on the stainless steel surface. The results are shown in Table 2. The surface contact resistance became 20 mΩ · cm ² or less by the surface modification treatment in test symbols 2, 3, 6, 7, 9-13, and 16 in which the thickness of the oxide film measured by AES was in the range of 20 to 600 nm. .

The spectrum of each element obtained by photoelectron spectroscopy (XPS) for the atomic number concentration of Si, Al, Mn, Fe contained in the oxide film before the surface modification treatment is separated into an oxide peak and other peaks, It was determined by dividing the peak area of the oxide by the relative sensitivity coefficient. The surface contact resistance is 20 mΩ · cm ² or less due to the surface modification treatment, and the atomic number ratio (Si + Al + Mn) / Fe contained in the oxide film is 1.0 or less in test symbols 2, 3, 6, 7, and 9-13. there were.

C: 0.003% by mass, Si: 0.08%, Mn: 0.11%, Al: 0.04%, Cr: 29.0%, N: 0.009%, Nb: 0.18 %, Mo: Stainless steel containing 1.99% was melted to obtain a stainless steel slab having a thickness of 200 mm. The obtained stainless slab is heated to 1200 ° C., hot rolled to a plate thickness of 2.0 mm at a finishing temperature of 900 ° C. and a coiling temperature of 650 ° C., and hot-rolled sheet annealed at a temperature of 1100 ° C. Next, cold rolling and cold-rolled sheet annealing at 1050 ° C. were repeated a plurality of times to obtain a stainless steel strip having a width of 1080 mm, a length of 1100 m, and a thickness of 0.1 mm. In addition, the cold-rolled sheet annealing was not performed after the last cold rolling. The obtained stainless steel strip was further divided into a width of 200 mm to form five steel strips, and each was heat-treated under the heat treatment conditions shown in Table 3. Then, a steel strip is passed through the steel strip in a 5% HNO ₃ -10% HF acid solution at 60 ° C. so that the immersion time is 180 s, and the surface contact resistance is reduced at intervals of 50 m from the tip of the steel strip. It was measured. The measurement pressure of the surface contact resistance was 1 MPa. FIG. 3 shows changes in surface contact resistance depending on the measurement position. In Examples 17 to 19 of the present invention, a low surface contact resistance was obtained uniformly from the coil front end to the terminal end. On the other hand, in Comparative Examples 20 and 21, the surface contact resistance increased from the middle of the coil to a value that could not be used as a separator. This is because metal ions were eluted in the acid solution due to the passage of the coil to the acid solution tank, and the reactivity of the acid solution was lowered. Therefore, in Comparative Examples 20 and 21, the removal of the oxide film was insufficient. It is thought that the surface contact resistance increased.

  The stainless steel of the present invention is suitable as a stainless steel for polymer electrolyte fuel cell separators, and is also suitable as a stainless steel for current-carrying members of various electric devices.

Claims (7)

In mass%, C: 0.001 to 0.10%, Si: 0.001 to 1.0%, Mn: 0.001 to 1.2%, Al: 0.001 to 0.5%, Cr: 15.0-35.0% N: containing 0.001 to 0.10%, the balance being Fe and unavoidable impurities, Ri thickness. 20 to 500 nm der oxide film on the surface, the surface A stainless steel having a contact resistance of 20 mΩ · cm ² or less.   Further, in terms of mass%, Ti: 1.0% or less, Nb: 1.0% or less, Zr: 1.0% or less, Cu: 1.0% or less, V: 1.0% or less, Ni: 12. The stainless steel according to claim 1, containing one or more of 0% or less and Mo: 5.0% or less.   3. The stainless steel according to claim 1, wherein the atomic ratio of Si, Al, Mn, and Fe contained in the oxide film satisfies (Si + Al + Mn) /Fe≦1.0. The separator for polymer electrolyte fuel cells which consists of stainless steel of any one of Claims 1-3 . In producing the stainless steel according to any one of claims 1 to 3, after cold rolling or after cold rolling material annealing, the hydrogen concentration is 30% by volume or more, and the balance is inert gas and inevitable impurities. A stainless steel, characterized in that a heat treatment is performed at a temperature of 800 to 1200 ° C. in an atmosphere having a dew point of −40 to 0 ° C., and a surface modification treatment using an acid solution is performed after the heat treatment. Method. The method for producing stainless steel according to claim 5 , wherein the acid solution is an inorganic acid. The method for producing stainless steel according to claim 6 , wherein the inorganic acid is hydrofluoric acid or hydrofluoric nitric acid.

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